1. Field of the Invention
The present invention relates to a current sensor capable of measuring target current in a contactless manner.
2. Description of the Related Art
In the fields of electric-powered vehicles and solar batteries, there has been known a current sensor for detecting an induced magnetic field, resulting from target current to be measured, using a giant magnetoresistive (GMR) element, serving as a magnetic detecting element (refer to U.S. Pat. No. 7,394,240, for example). The current sensor disclosed in U.S. Pat. No. 7,394,240 detects target current on the basis of the difference between outputs of a plurality of GMR elements, thus achieving high sensitivity.
The GMR element fundamentally includes an antiferromagnetic layer, a fixed magnetic layer, a nonmagnetic layer, and a free magnetic layer. The fixed magnetic layer is disposed on the antiferromagnetic layer and the direction of magnetization of the fixed magnetic layer is fixed in a given direction by an exchange-coupled magnetic field generated between the fixed magnetic layer and the antiferromagnetic layer. The free magnetic layer is disposed on the fixed magnetic layer with the nonmagnetic layer (nonmagnetic interlayer) therebetween. The magnetization direction of the free magnetic layer changes depending on an external magnetic field. The GMR element is configured such that its electrical resistance varies depending on the relationship between the magnetization direction of the free magnetic layer depending on the external magnetic field applied and the magnetization direction of the fixed magnetic layer. Thus, the external magnetic field can be detected.
To increase the linearity of the relationship between the electrical resistance and the intensity of the external magnetic field, such a GMR element may include a hard bias layer to apply a bias magnetic field to the free magnetic layer. Consequently, the magnetization of the free magnetic layer is aligned in a given direction, thus increasing sensitivity to detect the external magnetic field. A current sensor with higher detection sensitivity can be achieved using such a GMR element (refer to Japanese Unexamined Patent Application Publication No. 2006-66821, for example).
For the above-described GMR element including the hard bias layer to apply the bias magnetic field, the direction of magnetization of the free magnetic layer in the GMR element is aligned by the bias magnetic field. If an external magnetic field is applied in a direction parallel to the bias magnetic field (or perpendicular to a sensing direction of the GMR element), the detection sensitivity would change. In a current sensor including this GMR element, the accuracy of current measurement may decrease due to the presence of an external magnetic field oriented in the direction parallel to the bias magnetic field.